Magnesium materials are important light weight structural materials in vehicle technology, in engine construction, in aviation and space technology and in any other light weight construction. Due to the low specific weight of magnesium combined with very good strength characteristics, a noticeable weight reduction of the structural magnesium components is possible compared to aluminum or steel components. Compared to aluminum materials magnesium alloys have a noticeably better castability, which leads to a reduction in process steps and in an increase in productivity. Particularly, it is possible, in contrast to aluminum materials, to produce very complex, thin walled magnesium components while casting high production numbers. The use of magnesium materials in transporting means opens a high potential for cost reduction, fuel saving and payload increase.
The energy required for the primary production of magnesium is quite competitive compared with the energy required for the primary production of aluminum. In connection with re-use of magnesium only 5% of the energy needed for the primary production are required. Recycling concepts as employed for aluminum materials would thus lead to an significant reduction of the energy costs in connection with magnesium materials. However, even if no recycling is performed, it is easy to introduce magnesium materials back into the natural cycle of valuable materials.
However, the corrosion characteristic of magnesium materials is seen as a hindrance to their use. Water containing corrosive media primarily halogenate containing aqueous corrosion media can substantially influence the function of components made of magnesium. Due to this magnesium characteristic, the reluctance to use magnesium materials, particularly in aviation and space technology is very high. Even in vehicle technologies the corrosion characteristic of components subject to high loads and critical to safety plays a decisive role for example for crush elements.
Magnesium is a so-called "valve metal" which means it is capable of passivating itself. However, the passivating characteristic of magnesium is for example not as good as that of aluminum, because the grid structure of the magnesium hydroxide layer forming itself is geometrically smaller than the grid structure of the magnesium metal, whereby the protection layer can rip open. The natural passivating or protection layer of the magnesium is hardly stable against the attack of aggressive ions such as chlorides, because the chlorides can enter into the passivating layer thereby increasing its solubility.
In order to increase the corrosion resistance of magnesium structural components it is known to provide these components with so-called conversion layers in which cromates (VI) ions are embedded into the surface of the structural components. Further, an anodizing of the magnesium components is performed for example with the so-called "Magoxide" method. However, the conversion layers and the anodizing of the structural magnesium component lead only to a passivation of the component surface. This means that damage to the passivated surface layer causes the corrosion protection layer to fail at the point of damage of the magnesium structural component. The same problem occurs in connection with insulations such as organic coatings or insulation rings which are also used as corrosion protection for magnesium materials.
U.S. Pat. No. 4,770,946 discloses a magnesium material having, in addition to an oxide layer applied to the material, two resin layers and two metal layers forming a corrosion protection layer.
Japanese Patent Publication 4-297542 discloses a fiber reinforced composite magenesium material to which is applied a titanium or aluminim layer for corrosion protection purposes.
Particularly cathodic contaminations such as iron, nickel and copper have an adverse influence on the corrosion characteristic of magnesium materials. The amount of cathodic contaminations has been reduced to a minimum since the development of highly pure magnesium alloys. However, these elements can be present as a contamination of the component surface during manufacture and during working of the magnesium component for example due to chips or wear of the machining tool. Further, due to its position in the electro-chemical voltage series, magnesium tends to form contact corrosion with all metallic structural materials.